Topology and Control of Current-Fed Quasi Z-Source Inverter

Muneeb Ahmed. Int. Journal of Engineering Research and Applications www.ijera.com
ISSN: 2248-9622, Vol. 6, Issue 5, (Part - 6) May 2016, pp.14-19
www.ijera.com 14 | P a g e
Topology and Control of Current-Fed Quasi Z-Source Inverter
Muneeb Ahmed, Xupeng Fang
College of Electrical Engineering and Automation
Shandong University of Science and Technology, China
ABSTRACT
Quasi Z-source inverter is improvement to traditional Z-source inverter. Current-fed quasi Z-source inverter
(CF-QZSI) is an enhancement to Z-source inverter (ZSIs), it owns lower component rating, decreased source
stress, decreased component compute and prosaic control synthesis. With its distinct structure, the CF-QZSI can
operate the traditional zero states to buck the output voltage, which improves the inverter dependability greatly,
and provides a tantalizing single stage dc-ac conversion that is able to buck and boost the voltage. For dedications
with a variable input voltage, this inverter is a very competitive topology. The paper presents a comprehensive
study on the new features of CF-QZSI which include the advantageous buck-boost function, improved reliability
and reduced passive component rating, its characteristics is verified by the simulation results .
Keywords- Current-source inverter, Voltage source inverter, Current-fed quasi Z-source inverter, buck-boost.
I. INTRODUCTION
The voltage source inverter and current
source inverter provide an attractive single-stage dc-ac
conversion that is able to buck or boost voltage,
increase efficiency and reduce cost. However,
traditional inverters have drawbacks, i.e. behave in a
boost or buck operation only, and thus the obtainable
output voltage range is limited, either lower or higher
than the input voltage. The main switching device of
VSI and CSI are not interchangeable, and the
capacitor passes through high voltage. Z-source
inverter can overcome the inherent drawbacks of the
traditional inverters. The quasi Z-source inverter
(qZSI) is the improvement to traditional Z-source
inverter, voltage-fed qZSI have more attention than
the current-fed qZSI. The main drawback of
current-fed Z-source inverter is that the inductor
passes through high current.
The traditional current-fed quasi Z-source
inverter uses dc current source as the input. The dc
current source can be created by using uncontrollable
diode rectifier, battery and fuel-cell series an inductor.
Six switches are used in the traditional three-phase
inverter. Semiconductor devices are used as the
switches.SCR or power transistor with a series diode
can be used to provide unidirectional current flow and
bidirectional blocking. Newly developed switches the
reverse blocking IGBT (RB-IGBT) also promotes the
research on CSI [9], [10].
If we compare to current source inverter (CSI),
the standard voltage source inverter have 8 switching
states, including 6 active states and two zero states.
When the upper three or lower three switches are gated
on, shorting the load terminals. Current source inverter
have 9 nine valid states, 6 active and three zero state.
The three zero states produce zero ac line currents. In
this case, the dc-link current free wheels through either
the switches pole. The remaining states produce
non-zero ac output line currents.
This paper mainly focuses on the new feature of
current-fed quasi Z-source inverter, especially the
switching technique.
II. CURRENT-FED QZSI CIRCUIT
ANALYSIS
To improve the traditional ZSIs, four new
quasi-Z-source inverters, have been developed which
feature several improvements when compared to the
traditional ZSIs. They are voltage-fed qZSI with
continuous and discontinuous input current, current-fed
qZSI with continuous and discontinuous input current.
The current fed qZSI in a manner consistent with the
current-fed ZSI, are bidirectional with the diode, D. The
qZSI shown in Fig. 1, features reduced current in
inductor L2 and L3, as well as reduced passive
component count. Again, due to the input inductor, L1,
the qZSI in Fig. 1 do not require input capacitance. All
four qZSI topologies also feature a common dc rail
between the source and the inverter bridge, unlike the
traditional ZSI circuits. Furthermore, these qZSI
circuits have no disadvantages when compared to the
traditional ZSI topologies. These qZSI topologies
therefore can be used in any application in which the
ZSI would traditionally be used.
L1
L2 C1
C2
L3
D
C3 C4 C5
DC current
Source
LRRL
Quasi Z-source network Three-phase Inverter
Three-phase Load
IGBT1 IGBT3 IGBT5
IGBT4 IGBT6 IGBT2
D1 D3 D5
D4 D6 D2
inI
1LI
2LI
pnI
pnV
. Fig 1 current-fed qZSI with continuous input current
RESEARCH ARTICLE OPEN ACCESS

A.Active state
In active mode only one upper device and one
lower device which lies not same phase are conducted
simultaneously. In the active mode the inverter works
as traditional CSI.
The inverter bridge, viewed from the DC side
is equivalent to a current source, the input dc voltage is
available as dc-link voltage input to the inverter, which
makes the current-fed qZSI behave similar to a VSI. In
active state based on type of switch states the
dc-link voltage is equal to ac line voltage. So
Vpn=Vae. Fig 2(a) shows the active state equivalent
circuit.
B.Traditional zero state
In traditional zero state, the dc-link
voltage is zero (Vpn=0), the diode is OFF and the
switches block the ac output voltage. Fig. 2(b) shows
the short-zero state equivalent circuit.
C. Open zero state
Fig. 2(c) shows the equivalent circuit of the
open-zero state, the inverter bridge is equivalent to an
open circuit, the diode is ON and charges the capacitors
(C1, C2). The dc-link voltage is equal to sum of
Capacitors (V= VCl+VC2).
L3
L1
C1
C2
L2
Vin
lin
Vout
(a)
L3
L1
C1
C2
L2
Vin
lin
(b)
L3
L1
C1
C2
L2
Vin
lin
(c)
Fig 2(a) Active State, (b) Traditional zero state, (c)
open zero state
III. SPACE VECTOR PULSE WIDTH
MODULATION
IGBT is a gate controlled device; SVPWM
give path and control to AC Voltage. It technique use
for 3-phase inverter; ac output is sinusoidal and has
high adaptability. Any three functions of time that
gratify use space transformation.
ua(t)+ub(t)+uc(t)=0 (1)
Represented two dimensional space
2 2
3 3
2
( )
3
ej e j
a b c
u t u u u
 
   
   
   
 
   
 
 
(2)
2
3
is a scaling factor. if Va ,Vb and Vc are phase voltage
of a fair supply with a peak value of Vm
.sin ( )a m
u V t (3)
sin ( 2 / 3)b m
u V t   (4)
sin ( 2 / 3)c m
u V t   (5)
In the space vector illustration is
( )
j t
m
u t V e

 (6)
A. Switching technique
The current-fed QZSI has ten possible
switching states, of which three are traditional zero
state and six are active states and one of open zero state.
In open zero state all the switches of the inverter bridge
is turned off. Traditional zero state can be incept by
turning on an upper switch (S1, S3, and S5) and a lower
switch (S4, S6, and S2) from the same phase leg. Active
state can be incepting by turning on the switches from
different phase legs.

Table 1 shows the switching states of qZSI
Duty ratio are represented as active state (T1/T=DA),
short-zero state (T0/T=Dsh) and open zero state
(T2/T=Dop) [3].The switch states and inductor voltage
are illustrated as follows
DA+ Dsh+ Dop=1
(7)
( ) 0L A in o sh in o p in
v D V V D V D V     (8)
1 2 op
o in
A
u
V V
D

 (9)
Fig.3 shows the SVPWM of quasi z-source inverter.
Maximum value of modulation index is 2 / 3 , for
0<M<1 the inverter operates as normal SVPWM, when
M< 2 / 3 the inverter operates as over modulation.
The space vector pulse width modulation diagram is the
hexagon. The output power is given as
3 3
cos
2 2
o dc o
P M L V  (10)
Where V0 is the rms value of the output phase
voltage and Ф is phase angle between the output phase
voltage and the corresponding current. Thus by
selecting M value and switch states the buck-boost
expertise is realized.
I1
I2
I3
I4
I5
I6
II
III
V
VI
α
β
I9
I8
I7
Iref
θ
30°
IIV
Fig 3 six possible current vectors
Assuming that is in sector 1 as shown in
Fig.3, the duration’s t1, t2, and t0 can be obtained from
the following current-time integral:
21 1
21 1
1 2 00 0
s s
ref
t t TtT
t t t
d t d t d t d tI I I I


     
   
On the other hand the duration t0 = [Ts – (t1 + t2)]
For high switching frequency can be assumed
constant during each Ts time. I1, I2 are also
considered constant during each cell time while I0 =0
00
1 20 0
3 0 9 0
3 0 9 0
3 sref
S in S in S in
d c
C o s C o s C o s
I t t TI


      
       
      

Idc
S1
S2
S3
S4
S5
S6
t
Sector Time Ts
Fig.4 Tsector time switching Strategies
First sector switching sequence and duration
1 2 1 1 2 10 , 2 0 , 2
, , , ,
sec
and
F irst sector Second tor
and andI I I I I II I I I                   
        
1 2 1 2
0 0
, ,
1 12 2
, , , , , ,
sec sec
2 2 2 22 22 2
a n d
F irst to r S eco n d to r
t tt tt t t t
a n d a n dt t
                         
Second sector switching sequence
1 0 1 1 0 10 2 2 , 0
, , , ,
sec sec
an d a n d
F irst to r S eco n d to r
a n dI I I I I II I I I                   
        
I
II
III
IV
V
VI
I


1T
Ts
2T
Ts
1(100)I
2 (110)I3 (010)I
4 (010)I
5 (001)I 6 (101)I
Fig.5 Space vector of Qzsi
IV. SIMULATION RESULTS
The CF-QZSI can be operated in both boost
and buck operations depending on inductor value. If
L1=L2=20mH the current is high, output dc voltage is
low and voltage buck expertise is realized. Fig.6 shows
the simulation implementation of current-fed qZSI bust
State
No
Switch
state Type
S1 S4 S3 S6 S5 S2
1
Active
State
On Off Off On Off On
2 On Off On Off Off On
3 Off On On Off Off On
4 Off On On Off On Off
5 Off On Off On On Off
6 On Off Off On On Off
7
Traditional
zero state
On On
8 On On
9 On On
10 Open zero
state
Off Off Off Off Off Off
I ref

I ref


capability. When the input inductance value
L1=L2=150mH the input current is low, output dc
voltage is high and voltage boost expertise is realized.
The voltage and current blocks are amalgamated to
effect power buck capability.
Fig.5 or 6 shows the simulated power
buck-boost capability. Directly above, the theoretical
calculations are given as:
o in
V N V (11)
Vin is input dc voltage source of qZSI, V0 is dc output
voltage measured after the impedance network and N is
duty ratio of switch states where
1 2 o p
A
D
N
D


(12)
If we suppose the ac output voltage is Vac. Modulation
index is M, and thus we have
ac
I
0 .8 6 6
d c
I
m  (13)
The ac voltage and current blocks are multiplexed to get
power buck-boost aptitude. Impedance network
parameter are L1=L2=L3=20mh C1=C2=200uF and dc
input voltage 300V.
Boost operation results
Fig.6 Simulated boost capability of current-fed qZSI
Buck operation results
Fig.7 Simulation buck capability of current-fed qZSI
V. CONCLUSION
This paper deals with simple technique to
achieve power buck-boost capability. The advantages
of this technique are simple, efficient, and reduce
complexity. The current-fed integrated qZSI is
specially suited for hybrid vehicles and variable
speed motor drives. Unique features like single stage
power conversion, improved reliability, low EMI are
obtained. The current-fed qZSI concept can be easily
applied to adjustable-speed drive system. The
buck-boost operation is attained in simple procedure.
Better results are obtained through qZSI with
120-degree and space vector technique. The effects due
misfiring are overcome. Gating pulses for IGBTs are
contributing in merited procedure through SVPWM.
Two on-line PWM gating pattern generators for
three-phase current source converters have been
proposed. These are techniques best suited for analog
control schemes, which uses carrier signal and one best
suited for digital control schemes based on space
vector.
ACKNOWLEDGMENT
I would like to thank my advisor, Dr. Xupeng
Fang, for his guidance, encouragement, and support
during this work.
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